Colorado Supreme Court Holds the Clean Air Act Does Not Preempt State Tort Damages Claims for Climate-Related Harms Against Fossil Fuel Producers

Colorado Supreme Court Holds the Clean Air Act Does Not Preempt State Tort Damages Claims for Climate-Related Harms Against Fossil Fuel Producers

Introduction

In a closely watched climate litigation decision, the Colorado Supreme Court, sitting en banc, held that state common law tort claims for damages brought by Boulder County and the City of Boulder (collectively, “Boulder”) against Exxon Mobil Corporation and Suncor Energy entities are not preempted by federal law. The case, an original proceeding under C.A.R. 21, addresses whether Boulder's claims for public and private nuisance, trespass, unjust enrichment, and civil conspiracy—premised on the defendants’ production, promotion, refining, marketing, and sale of fossil fuels and alleged misrepresentations—may proceed in Colorado courts notwithstanding the Clean Air Act (CAA), federal common law, and the federal foreign affairs power.

The ruling arrives after years of jurisdictional skirmishing, including removal to federal court, remand affirmed by the Tenth Circuit, and renewed dismissal motions in state court. It squarely places Colorado alongside courts that have permitted similar climate-related tort actions to proceed under state law, notably the Hawai‘i Supreme Court in City and County of Honolulu v. Sunoco LP, while rejecting the Second Circuit’s contrary approach in City of New York v. Chevron Corp.

Case Snapshot

  • Case: In re County Commissioners of Boulder County and City of Boulder v. Suncor Energy USA, Inc., et al.; Exxon Mobil Corp.
  • Citation: 2025 CO 21, No. 24SA206 (Colo. May 12, 2025)
  • Court: Colorado Supreme Court, en banc
  • Author: Justice Gabriel (Chief Justice Márquez, Justices Hood, Hart, and Berken Kotter joining)
  • Dissent: Justice Samour, joined by Justice Boatright
  • Holding: Boulder's state tort claims are not preempted by federal law (neither federal common law nor the CAA), and they are not barred by the federal foreign affairs power.
  • Disposition: Order to show cause discharged; remanded for further proceedings; no view expressed on the merits.

Summary of the Opinion

The Court held that Boulder's state-law tort claims for damages are not preempted. It reasoned that:

  • Any federal common law of interstate pollution relevant here has been displaced by the CAA (following American Electric Power Co. v. Connecticut), so federal common law plays no role in the preemption analysis.
  • The CAA does not expressly preempt such state common law claims, does not occupy the field, and does not conflict with Boulder's damages claims; the Act contains savings clauses preserving state law and citizen rights.
  • Foreign affairs preemption does not apply; the claims do not intrude on or conflict with federal foreign policy.

The Court emphasized that Boulder seeks monetary relief only—no injunction, no emissions controls—and targets upstream production, promotion, and marketing conduct (including alleged misrepresentations), not the regulation of emissions themselves. The Court adopted the reasoning of the Hawai‘i Supreme Court in Honolulu and aligned with the Fourth Circuit’s analysis in Mayor & City Council of Baltimore v. BP P.L.C. It expressly rejected the Second Circuit’s “authorization” theory in City of New York v. Chevron, which treated displaced federal common law as retaining preemptive force absent statutory authorization of state claims.

Analysis

Precedents Cited and Their Influence

  • Erie R.R. Co. v. Tompkins (no “federal general common law”): Sets the baseline; only limited federal common law is recognized in specialized areas implicating uniquely federal interests.
  • Illinois v. City of Milwaukee (Milwaukee I) and City of Milwaukee v. Illinois (Milwaukee II): Established and then recognized the displacement of federal common law in water pollution by subsequent federal legislation; key for understanding “displacement” as removing federal common law from the field when Congress legislates.
  • American Electric Power Co. v. Connecticut (AEP): Held that the CAA displaces federal common law nuisance claims seeking to abate greenhouse gas emissions; directed courts to look to the CAA’s preemptive effect regarding state claims.
  • Native Village of Kivalina v. ExxonMobil: Reaffirmed that the CAA displaced federal common law greenhouse gas nuisance claims.
  • City & County of Honolulu v. Sunoco LP (Haw. 2023): Persuasive authority that claims like Boulder’s are not preempted by federal common law or the CAA; heavily relied on by the majority.
  • Mayor & City Council of Baltimore v. BP P.L.C. (4th Cir. 2022): Found no CAA preemption of analogous state tort damages claims; cited for both statutory and foreign affairs preemption analyses.
  • City of New York v. Chevron Corp. (2d Cir. 2021): The Court rejected its “backwards” analysis, declining to treat displaced federal common law as retaining preemptive force absent express statutory authorization.
  • Wyeth v. Levine; English v. General Electric; Geier v. Honda; Utility Air Regulatory Group v. EPA; Virginia Uranium, Inc. v. Warren: Provide the framework for preemption analysis (express, field, and conflict) and underscore the need for a concrete statutory basis for preemption rather than a “brooding federal interest.”
  • CAA savings clauses: 42 U.S.C. §§ 7416 and 7604(e) (citizen suit): Preserve state rights and stricter standards; central to rejecting field preemption.
  • Zschernig v. Miller; American Insurance Ass’n v. Garamendi; United States v. Pink; Hines v. Davidowitz: Define foreign affairs preemption; cited to show its limited, rarely invoked scope not reached by Boulder’s damages claims.

Legal Reasoning

1) Displacement of Federal Common Law and Its Consequence

The Court reaffirmed that the CAA displaced federal common law governing interstate pollution nuisance claims, citing AEP and related decisions. Displacement means federal common law no longer supplies the rule; preemption must be assessed under the statute. The Court rejected the notion that displaced federal common law somehow continues to bar state law unless Congress “authorizes” it. Instead, as AEP instructs, the availability of state law depends on the CAA’s preemptive effect—not on a separate “authorization” requirement.

The Court further noted that, even before displacement, federal common law applied most clearly to suits by states to abate pollution emitted by out-of-state sources; Boulder’s claims differ: they seek damages against upstream producers and marketers for their role in causing climate-related harms, including through alleged misrepresentations—not abatement of emissions. No Supreme Court case has applied federal common law in that posture.

2) The Clean Air Act: No Express, Field, or Conflict Preemption

Applying standard preemption principles under the Supremacy Clause:

  • Express preemption: The CAA contains no express preemption clause that reaches state common law damages claims of the sort Boulder brings.
  • Field preemption: Congress has not occupied the field. The CAA’s cooperative federalism regime contemplates substantial state roles through state implementation plans, and its savings clauses preserve stricter state standards and state/common-law remedies.
  • Conflict preemption:
    • Impossibility: Defendants identified no statutory or regulatory command that would make compliance with both the CAA and state tort duties physically impossible, nor that state law penalizes what federal law requires.
    • Obstacle: The Court found no interference with the CAA’s purposes. Boulder seeks damages, not injunctive emissions controls; the claims do not require courts to set emissions standards or police emitters. The Act itself places primary responsibility for air pollution control on states and local governments and contemplates citizen and state action.

At a high level, the majority emphasized that this is not a uniquely federal regulatory field, but rather one touching traditional state tort interests (nuisance, trespass, unjust enrichment, conspiracy), and that the CAA preserves state space rather than extinguishing it.

3) Damages Claims Are Not De Facto Emissions Regulation

The Court rejected the argument that damages are a form of de facto regulation that must be preempted. While acknowledging the general proposition that damages can influence conduct, the Court found it inapposite here: the CAA did not occupy the field, and equating any substantial damages exposure with regulation would effectively federalize large swaths of state tort law, contrary to precedent and the CAA’s savings clauses. The opinion distinguished Kurns (Locomotive Inspection Act field preemption case) and noted that lawsuits are not transformed into emissions regulation simply because they may affect industry behavior.

4) Foreign Affairs Preemption Does Not Bar the Claims

On foreign affairs preemption, the Court found:

  • Conflict preemption: Defendants identified no specific federal foreign policy that Boulder's claims would conflict with or obstruct.
  • Field preemption: The claims do not intrude into the field of foreign affairs or establish competing state foreign policy. Traditional tort law claims seeking local damages for local harms are a historic area of state responsibility and, at most, intersect with private international commerce.

The majority aligned with the Fourth Circuit’s Baltimore decision, viewing these suits as addressing traditional state interests without more than incidental effects on foreign affairs.

5) The Court’s Treatment of Divergent Authority

The Court expressly aligned with Honolulu (Haw. 2023) and Baltimore (4th Cir. 2022) and disagreed with the Second Circuit’s City of New York analysis. It criticized the New York panel’s “backwards” approach—asking first whether federal common law would have applied (and hence preempted state law), then holding that displacement by statute does not revive state law absent statutory authorization. The Colorado Supreme Court read AEP to require a statute-focused preemption analysis once displacement occurs, without any additional authorization requirement.

The Dissent’s Framework and Concerns

Justice Samour, joined by Justice Boatright, dissented. Key points:

  • Federal common law existed historically to govern interstate and international pollution because of uniquely federal interests and the need for uniformity. Displacement by the CAA did not suddenly make state law competent; instead, courts should ask whether the CAA affirmatively authorizes such state law claims.
  • Relying on Ouellette and City of New York, the dissent reads the CAA and similar schemes as preserving only limited state remedies, typically focused on source-state law, to avoid a patchwork of standards and regulatory chaos.
  • The dissent views Boulder’s damages claims as functionally aimed at abating or regulating cross-border emissions, despite the absence of an injunction request, and argues that such suits entail more than incidental effects on foreign affairs. It warns of interference with national policy and coordination (citing, by way of example, executive engagement with the Paris Agreement).
  • On this view, ordinary preemption analysis, with its presumption against preemption, is “ill-suited” because states have not historically occupied this field; instead, the question is whether Congress has authorized state remedies of this kind. The dissent would dismiss the claims.

The majority implicitly responded by emphasizing the CAA’s savings clauses, the traditional nature of tort remedies, the absence of actual conflict with express foreign policy, and the difference between damages actions against upstream conduct and abatement suits aimed at emitters.

Impact

On Colorado Litigation

  • State tort climate suits can proceed past preemption challenges in Colorado courts when they seek damages for local harms and do not demand emissions controls or injunctive relief.
  • The decision clears the way for discovery and merits litigation on causation, knowledge, foreseeability, damages allocation, and defenses (including those specific to misrepresentation and conspiracy theories).
  • The Court did not address personal jurisdiction or the sufficiency of the pleadings on elements other than preemption; those issues remain for the trial court.

On National Climate Litigation

  • The ruling contributes to a developing split: it aligns with Honolulu and Baltimore and departs from City of New York’s “authorization” approach. It adds a state supreme court voice to the majority trend allowing state tort claims to proceed.
  • The decision may encourage similar claims by other Colorado municipalities and may be cited persuasively elsewhere, particularly on the statutory preemption framework and the rejection of a categorical “damages equals regulation” argument.
  • Foreign affairs preemption arguments may face headwinds when plaintiffs seek local damages without attempting to set or enforce emissions standards.

On Preemption Doctrine

  • Doctrinally, the opinion underscores the distinction between “displacement” (removing federal common law) and “preemption” (statutory override of state law). After displacement, courts look solely to the statute, not to residual federal common law interests.
  • The presumption against preemption remains salient in fields of traditional state regulation; the Court characterizes nuisance, trespass, unjust enrichment, and conspiracy as such, even when the factual context is climate change.
  • The CAA’s savings clauses matter: they weigh against field preemption and undercut broad obstacle-preemption claims in damages suits not seeking emissions controls.

Complex Concepts Simplified

Displacement vs. Preemption

  • Displacement: When Congress legislates comprehensively in an area that courts had previously governed by federal common law, the statute “displaces” that federal common law. After displacement, federal common law no longer supplies the rule of decision.
  • Preemption: Whether federal statutory law overrides state law. Preemption can be express (Congress says so), field (Congress occupies the field), or conflict (impossibility or obstacle to federal purposes).
  • Key point here: Federal common law about greenhouse gas nuisance is displaced by the CAA; thus, courts must analyze preemption under the CAA, not by residual federal common law concerns.

Field vs. Conflict (Impossibility and Obstacle) Preemption

  • Field preemption: Congress leaves no room for state regulation in a given area.
  • Impossibility conflict preemption: It is impossible to comply with both state and federal law, or state law penalizes what federal law requires.
  • Obstacle conflict preemption: State law stands as an obstacle to Congress’s purposes. Courts apply this sparingly, typically in uniquely federal areas or when Congress deliberately struck a balance that state law would upset.
  • Here: The Court found no field preemption because the CAA contemplates and preserves state roles; no impossibility or obstacle because damages claims do not force emissions standards or conflict with CAA goals.

Foreign Affairs Preemption

  • Conflict-based: A state law cannot conflict with or obstruct federal foreign policy.
  • Field-based: States cannot intrude into the field of foreign affairs or establish their own foreign policy; usually invoked in rare cases where the state law has more than an incidental effect on foreign relations.
  • Here: The Court found neither type applies; common law damages claims for local harms do not set foreign policy or conflict with a specific federal policy.

Why Damages Are Not Automatically “Regulation”

  • Damages can influence conduct, but that alone does not convert a tort suit into regulatory action preempted by federal law.
  • When Congress has not occupied the field and has preserved state remedies, a damages action does not morph into an impermissible emissions control regime.

Key Takeaways

  • In Colorado, state-law tort damages claims seeking compensation for climate-related harms linked to fossil fuel production, promotion, refining, marketing, and sales may proceed; they are not preempted by federal common law or the CAA.
  • Displacement of federal common law by the CAA means courts evaluate preemption solely under the statute; there is no threshold requirement that Congress “authorize” state claims.
  • The CAA’s savings clauses and cooperative federalism structure weigh against field and obstacle preemption for damages claims that do not seek to set emissions standards.
  • Foreign affairs preemption does not bar these claims absent a clear conflict with federal policy or a state attempt to set foreign policy.
  • The Court did not opine on the merits; plaintiffs must still establish causation, damages, and other elements, and defendants may raise all other defenses.

Open Questions and What Comes Next

  • Merits litigation will likely involve complex causation and attribution issues, including apportionment, knowledge and misrepresentation evidence, and foreseeability.
  • Choice-of-law and extraterritoriality issues may arise as the case proceeds, particularly given the breadth of defendants’ alleged global conduct.
  • Defendants may pursue other defenses (e.g., personal jurisdiction, statute of limitations, First Amendment issues regarding advertising allegations), none of which were resolved here.

Conclusion

The Colorado Supreme Court’s decision sets a significant state-level precedent in climate tort litigation: once the CAA displaced federal common law, the preemption inquiry is statutory, and on that score the CAA neither expressly nor impliedly preempts Colorado tort damages claims that do not seek to regulate emissions. By rejecting the “authorization” approach and foreign affairs preemption arguments and by embracing the CAA’s savings clauses and cooperative federalism, the Court preserves space for traditional state remedies to address local harms allegedly caused by global fossil fuel commerce and communication. The ruling does not resolve whether Boulder can ultimately prevail, but it ensures that, in Colorado, the courthouse doors remain open for such claims to be tested on their merits.

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